1. Field of the Invention
The present invention relates to a touch signal probe for measuring a configuration of a workpiece by a coordinate measuring machine and the like.
2. Description of Related Art
One-dimensional measuring machine, a height gauge for instance, a coordinate measuring machine and a profile measuring machine for measuring configuration of solid body are known as a measuring machine for measuring a configuration and a dimension of a workpiece. Various probes are attached to the measuring machine to detect relative position of a measuring machine body and the workpiece.
The probes are classified into non-contact type probes and contact-type probes, or into continuously measuring probe and touch trigger probe etc.
An ultrasonic touch signal probe disclosed in Japanese Patent Publication Laid-open No. Hei 6-221806 is known as an example of the above-mentioned contact type touch trigger probe for the coordinate measuring machine.
As shown in FIG. 11, a touch signal probe 100 includes a stylus holder 101, a vibrator 102 and piezoelectric element 103 etc. The stylus holder 101 is attached to a movable shaft of a coordinate measuring machine etc (not shown). A contact to the workpiece is detected while the movable axis is moved and the coordinate when the probe touches the workpiece is read, thereby measuring the configuration etc. of the workpiece.
The stylus holder 101 is configured in a hollow cylindrical shape and has the vibrator 102 at a lower inner end (a direction in the figure, equally applied hereinafter) thereof supported by a pair of engage pin 104 disposed on a support point arranged at a substantially central portion in the axial direction of the vibrator 102. A contact ball 102A to be abutted to the workpiece during measurement is mounted on a lower end of the vibrator 102 and a counter balance 102B having the same weight as the contact ball 102A is attached on an upper end of the vibrator 102, so that the support point to the stylus holder 101 is consistent with a centroid of the vibrator 102.
A pair of groove portion 105 formed by cut-out is provided at an outer circumference of the vibrator 102 and the piezoelectric elements 103 are attached spanning over the respective groove portions 105. The pair of groove portion 105 and the pair of the piezoelectric element 103 respectively have the same configuration. Both ends of the respective piezoelectric elements 103 are fixed at two points of the outer circumference of the vibrator 102 across the groove portion 105 by an adhesive etc.
The piezoelectric elements 103 are disposed substantially symmetrically at a center of the support point of the vibrator 102 along the axial direction of the vibrator 102 and are divided into a vibrating means 103A for vibrating the vibrator 102 resonantly and a detecting means 103B for detecting a change in the vibration of the vibrator 102.
In the touch signal probe 100, a node of vibration of the vibrator 102 coincide with the support point when the vibrator 102 is vibrated along the axial direction thereof by the vibrating means 103A.
According to the touch signal probe 100, since the centroid of the vibrator 102 is supported by the stylus holder 101 at a portion of the node of vibration, stability against disturbance vibration is improved to enhance measurement accuracy of the touch signal probe.
However, following problems occur in reducing an entire size of the probe in thus-structured conventional touch signal probe 100.
When the size of the entire touch signal probe 100 is reduced, the size of the support point of the vibrator 102 is reduced in proportion to the entire dimension. In other words, in order to prevent the support point continuous with a non-vibrated stationary portion from restraining the vibration of the vibrator 102, the support point is required to be positioned within a predetermined area adjacent to the node of vibration of the vibrator 102 and the size of the area is lessened when the size of the touch signal probe 100 is reduced, which result in difficulty in supporting the vibrator 102 on account of minute arrangement thereof.
Further, since the vibrating means 103A and the detecting means 103B mounted with the node of vibration therebetween are difficult to be disposed, highly accurate processing and assembly of the components with high accuracy can be difficult.
On the other hand, a vibrator shown in U.S. Pat. No. 5,524,354 is known as a vibrator for a touch trigger probe aiming at reduction in size.
As shown in FIG. 12, a vibrator 202 has a tuning-folk shaped vibrating portion 203, a stick stylus 204 disposed at a pointed end of the tuning-folk shaped vibrating portion 203, and a tip ball 205 disposed at a pointed end of the stylus 204. The respective styluses 204 are arranged to cause flexural vibration in a direction orthogonal with an axial direction thereof in synchronization with the vibration of the tuning-folk shaped vibrating portion 203. Base portion of the tuning-folk shaped vibrating portion 203 does not vibrate and support thereof is easy even when the size of the vibrator 202 is small, and a vibration property at a distal end can be enhanced to enable high sensitivity. Accordingly, the reduction of the size of the touch trigger probe can be easily attained while maintaining high sensitivity thereof.
In the touch trigger probe utilizing the vibrator 202, however, since the stylus 204 causes flexural vibration in synchronization with the vibration of the tuning-folk shaped vibrating portion 203, the sensitivity can be widely varied according to which part of the tip ball 205 contacts the workpiece (having direction dependence). Further, since the vibration range of the distal portion of the stylus 204 is generally set large, the accuracy cannot be obtained as compared to ordinary touch trigger probe.